1. Field of the Invention
The present invention relates to a magnetooptic head which can record, read, and erase information on or from a magnetooptic disk.
2. Description of the Prior Art
So far, various optical disk memories have been developed as memory units of higher density and large capacitance. In parallel to the research and development of the disks, optical disk drives have been developed, of course. However, the higher density and larger capacitance of the disk memory are greatly dependent upon the optical head memory. In other words, it is no exaggeration to say that the head decides the value of the memory unit.
The optical head is disclosed in some Patent Applications. For instance, Japanese published Unexamined Patent Appli. (Kokai) No. 58-12145 discloses a head in which a permanent magnet is fixed to a condenser lens and coils are mounted on a fixed base near the permanent magnet to drive the magnet in at least two directions for providing tracking and focusing control, independently. Further, Japanese Patent disclosure (Kokai) No. 59-221839 discloses a head in which coils are bonded to a condenser lens and a permanent magnet is fixed to a fixed base to drive the coils in at least two directions for providing tracking and focusing control, independently.
In the above-mentioned write-once type optical disk, information once written is not erasable or overwritable. However, it is of course desirable to erase unnecessary information from a disk to effectively utilize the optical disk.
Therefore, an overwritable magnetooptic disk memory has been developed and highlighted. The disk memory of this type will be described hereinbelow in brief.
To record information signals on the magnetooptic disk, a 1 .mu.m-dia. laser spot light is focused locally on the disk under an external magnetic field of 500 Oe or less to raise local temperature on the magnetic film and thereby to change the local magnetization direction in parallel to the external magnetic field. In this case; a pit of a 0.6 to 1 .mu.m width and a 1 to 2 .mu.m length is formed on the disk. To erase information signals from the disk, another external magnetic field opposite to the recording external field in direction is applied to the disk. To read information signals from the disk, the disk is irradiated with a linearly polarized laser beam to detect a difference in polarization direction of reflected or transmitted light between areas at which magnetization direction is inversed and those at which not inversed.
As described above, in order to record, read and erase information to or from the disk, external magnetic fields are necessary in addition to a laser beam for locally heating the magnetic film.
FIG. 1(A) shows an example of prior-art external magnetic field generating means disclosed in the two already-mentioned Japanese patent Applications, in which an optical head OH and a permanent magnet PM (or an electromagnet) are disposed on both sides of a disk D in opposing positional relationship to each other.
In the above arrangement, it is desirable that the intensity of the external magnetic field is substantially constant in both information recording and erasing operations. However, whenever the disk rotates, since the disk is subjected to dynamic axial runout, the distance between the disk D and the permanent magnet PM changes, so that the intensity of the external magnetic field on the disk fluctuates.
The fluctuations of the external magnetic field intensity cause variations in recorded pit size or in erased track width, thus resulting in drawbacks such that information signal quality, that is, disk drive reliability is deteriorated.
To overcome the above fluctuations, although it is possible to increase the distance between the disk and the external magnetic field generating means, the generating means may become large in size and another problem with heat generation may arise in the case of an electromagent.
To overcome the above-mentioned problem, Japanese Published Unexamined Patent Applic. (Kokai) No. 61-96540 discloses a head including a movable magnet as shown in FIG. 1(B). In this head, a cylindrical magnet MG is fixed to a condenser lens CL for focusing a laser beam LB onto a surface of a disk D, coaxially with the condenser lens CL. The magnet MG is adjustably moved to or away from the disk D by a focusing coil FC, together with the condenser lens.
In the above-mentioned prior art head, however, there exists another problem in that another tracking coil and another tracking magnet should be arranged within the same head, thus resulting in a relatively complicated mechanism, an increase in size and thickness of the head, an interference in magnetic field between focusing magnet and tracking magnet.
In the optical disk device of large capacity and high density, it is indispensable to implement focusing control and tracking control for the condenser lens. For doing this, a tracking guide groove called pregroove is formed on the disk in the concentric or spiral fashion with a track pitch of about 1.6 to 2 .mu.m. The condenser lens is controllably driven along the pregroove (in the tracking direction) on the basis of a tracking signal detected from the groove.
In the above-mentioned track access operation of the laser beam, the head is usually controlled in accordance with two, coarse and fine, operations. That is, when the access distance for tracking control is 50 to 60 .mu.m or more, the head is coarsely moved by an external actuator such as a linear motor; while when the access distance is 50 to 60 .mu.m or less, only the condenser lens is finely moved by an internal actuator housed within the head.
In the coarse track access operation of the above two-stage servo tracking system, the condenser lens is locked at a predetermined (central) position within the head by passing a current through the tracking coils in order to prevent vibrations of the condenser lens caused when the head is stopped near a designated track and to enter the succeeding fine track access operation immediately. On the other hand, in the fine tracking operation, the condenser lens is located at the central position within the head. In summary, the tracking operation can be achieved by a fine tracking actuator provided inside the head and a coarse linear actuator provided outside the head in combination for providing a high speed track access operation. Therefore, it is necessary to accurately detect the position of the condenser lens relative to the head especially for two stage tracking servosystem.
To detect the condenser lens position, conventionally two, reflected light and transmitted light, detection methods have been used.
FIG. 1(C) shows a reflected light detection method, by which light emitted from a light emitting diode LED is guided through a lens LS, reflected by a mirror M attached on a holder of a condenser lens CL, and then received by a two-element photodetector PD to detect unbalance between the two. In this case, the position of the condenser lens CL can be detected on the basis of a differential output between two output signals generated from the two-element photodetector PD.
FIG. 1(D) shows a transmitted light detection method, by which light emitted from an LED is guided through a lens LS, transmitted through a slit SL and then received by a two-element photodetector PD to detect an unbalance between the two caused when the slit SL moves.
In the prior-art condenser lens position detecting apparatus, there exist various drawbacks such that optical elements are required to be located at accurate positions relative to the lens holder; the number of parts is large; the movable condenser lens is unbalanced in weight because some additional elements are attached onto the lens holder, thus resulting in an inclination of the lens actuator or unstability thereof.